Process and apparatus for supplementary cooling of the center of a continuously cast metal bar

ABSTRACT

An improved process and device for the continuous casting of metal bars wherein an inert gas is insufflated into the liquid metal in the mould through a metal pipe dipped into said liquid metal and having substantially the same composition of the said liquid metal.

United States Patent 1 [111 3,833,047

Tovini Sept. 3, 1974 [54] PROCESS AND APPARATUS FOR 3,208,l l7 9/1965 (ioedecke et a1 164/259 X SUPPLEMENTARY COOLING OF THE 3,746,070 7/1973 Hill 164/57 CENTER OF A CONTINUOUSLY CAST METAL BAR Raffaele Tovini, Milan, Italy Dalmine S.p.A., Milan, Italy Nov. 9, 1972 Inventor:

Assignee:

Filed:

Appl. No.:

Foreign Application Priority Data Nov. 13,1971 Italy 31065/71 U.S. Cl 164/66, 164/82, 164/275, 164/281 Int. Cl B22d 11/10 Field of Search 164/57, 66, 82, 86, 275, 164/281 References Cited UNITED STATES PATENTS 10/1944 Hopkins 164/252 FOREIGN PATENTS OR APPLICATIONS 1,280,985 11/1961 France 164/275 1,587,288 3/1970 France 164/82 7/1961 Germany 164/66 Primary Examiner-R. Spencer Annear Attorney, Agent, or Firm-Kenyon & Kenyon Reilly Carr & Chapin 3 Claims, 2 Drawing Figures PROCESS AND APPARATUS FOR SUPPLEMENTARY COOLING OF THE CENTER OF A CONTINUOUSLY CAST METAL BAR BACKGROUND OF THE INVENTION The present invention concerns an improved process and device for the continuous casting of bars, of the type in which a metal, such as steel, is transformed from the melted state to the solid bar state through passage through a chilled mould. More especially the in vention applies to the processes for continuous casting in which the liquid metal is caused to flow into a mould of rectilinear axis which is substantially vertical;

It is known that metal bars produced by all continuous casting processes of known technique contain a porous zone in the center, which is normally no problem when the bar is used for rolled sections or structural shapes, due to the fact that the small internal holes fuse together as a result of the rolling pressure, but which instead can cause considerable inconveniences when the bar, especially in the round section, is intended for the manufacture of pipes. in that case in fact the first operation performed on the bar is that of piercing the center in a piercing rolling-mill using skew rolls which cause stresses in the center of the piece. These are not pressure stresses alone and therefore can open up the little internal holes causing subsequently defects such as scales on the internal surface of the manufactured pipe.

It is further known that in the interior of the bar which is solidifying there is a well of liquid metal, substantially in the form of an inverted cone, whose depth depends on the dimensions of the bar, on the pouring conditions of the melted metal and on the various parameters of solidification, such as the speed of extraction, intensity of the primary and secondary cooling, and so on. The greater the'depth of the liquid well, or the height of its cone, the more acute is the angle to the vertex of that cone, assuming a given bar section in the melted state at the entrance of the mould. It has been found that near the vertex the crystals that grow from the walls of the bar toward the interior meet forming bridges which can hinder the flow of the liquid metal into the underlying areas when holes form in those areas as a result of shrinkage. The excessive depth of the liquid well therefore turns out to be the principal reason for the formation of the above porosity in the center of the bar produced by continuous casting.

It has already been proposed an improved process for manufacturing metal bars through continuous casting which had as its aim the avoidance of the abovementioned inconveniences producing in the liquid metal in the act of solidifying an initial cooling in the center. which is added to the cooling performed on the exterior of the mould. According to the said known process, draught of inert gas is insufflated into the liq- SUMMARY OF THE INVENTION The object of the process according to this invention is to further improve the cooling conditions and the solidification of the billet during its formation in its passage through the chilled mould and through the subsequent secondary cooling zone. In particular this improved process allows to substantially reduce the porosity in the center of the bar by reducing the depth of 'the liquid well formed in its interior through a lowering of the temperature in the center area itself.

The above object is achieved through an improved casting device wherein the inert gas is insufflated into the melted mass through a piping which ends into a pipe made of metal whose composition is identical to that of the billet in course of solidification. This metallic pipe which wears out because it melts into the liquid mass in course of solidification, is introduced continuously from the top along the axis of the mould. The dimensions of this metal pipe as well as the speed with which it is fed are calculated in such a manner that the quantity of metal fed is completely melted before solidifying again. For melting the metal pipe is made use of the enthalpy of overheating of the surrounding metal mass which is in such manner further cooled. This supplementary cooling due to the melting of the metal pipe is, as well known to the skilled in the art, more intense than the caused by the insufflated gas stream.

The use of the metal pipe according to the present invention, instead of the refractory pipe of the prior art. offers another advantage because the metal pipe does not require a porous plug or diaphragm at its end. It is known in fact that such porous plugs or diaphragms are subject to occlusions due to momentary decreases of the pressure of the insufflated gas with subsequent penetration and solidification of the liquid metal into the pores of the plug or diaphragm.

BRIEF DESCRlPTlON OF THE DRAWINGS These and other advantages, objects and characteristics of the process and device of the present invention will become evident to the skilled in the art from the following detailed description of one embodiment referring to the attached drawings in which:

FIG. 1 is a schematic sectional view of an apparatus for continuous castingaccording to the process of the present invention; and

FIG. 2 shows in correspondence of two different transverse sections of the bar in the act of solidifying, and the temperature trends in the continuous casting process of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the liquid metal, particularly steel. contained in the crucible 1 is discharged into hopper 2 from which pours out a jet 3 which falls into mould 4 forming a liquid bath or well 5. Rolls 6 guide the bar partially solidified 7 through a curtain of water sprays produced by a series of nozzles 8 mounted on a manifold 9. The liquid well 5, of substantially invertedcone form whose base is the top free surface of the melted metal, has its lower vertex 10 at the point where the solidification is completed. Rolls 11 draw from the mould 4 the bar 7 which is subjected to secondary coolmg.

In FIG. 1, which shows a primary realizable method of the process covered by the present invention, hopper 2 is displaced in such a manner that jet 3 falls into the mould in off-center position, but could also provide for two or more jets symmetrical in respect to the axis of 5 the mould. The solid metal pipe 13, having the same composition as that of the melted metal and therefore of bar 7, is fed continuously through feed-rolls 12 into the center of liquid well 5. As stated previously, bar 13 also melts assorbing heat from the surrounding melted metal in the liquid well which for that reason becomes cooled. This further reduction of the temperature at the center of the solidifying bar results in a shortening of the liquid well 5 whose vertex will be therefore at a higher point and with a greater angular opening with respect to an analogous representation of the solidiflcation in a continuous casting process of the prior art. lnside the pipe 13 flows in direction 14 an inert gas which escapes toward the bottom of the area where the pipe is melting and then re-ascends toward the surface facilitating still more the complete melting of the tubular element 13 and directly cooling the center of the melted metal.

Any gaseous substance at room temperature, which neither reacts with the liquid metal nor modifies the metal properties of the melted mass, can be used as an inert gas according to the present invention. For example nitrogen or argon can be used to this purpose. The quantity of inert gas to be insufflated into the inner part of the melted mass is approprietaly chosen according to the volume of the melted mass contained in the mould, according to the extraction speed of the solidifled bar as well as according to other factors known to those skilled in the art. A stream of nitrogen between 0.3 and 1.6 liter/minute has given satisfactory results in many cases.

The lower end of the pipe 13 is dipped into the inner part of the melted mass down to the desired depth. which is usually in the range between about and about 80 centimeters. The metal pipe 13 is used in rolls of a considerable length. During the casting process the pipe 13 is first unrolled and then straightened by means of a known device which also provides for the continuous feeding of the pipe as its dipped end melts in the liquid well. The opposite end of metal pipe 13 is connected to the gas feeding piping through a known gastight rotative joint. The source of the fed inert gas can be a pressure tank or a plurality of metal bottles.

In FIG. 2, where below and in line with a schematic view in longitudinal section of a bar in the act solidifying according to the process of the present invention are the diagrams of the temperatures respectively in transverse sections AA and BB, with T there has been indicated the temperature corresponding to the change from liquid state to solid state.

From the diagram it is evident that at the central point M of section BB the temperature is lower than that at point N thus causing a more rapid solidification in the axial area of the bar.

Although the invention has been disclosed in detail with reference to an embodiment thereof, it is to be understood that possible additions and/or modifications can be made by those skilled in the art remaining in the scope of the present invention.

What I claim is:

l. A process for the continuous casting of metal in a vertical mold comprising the steps of feeding melted metal into the top of the mold,

at least partially solidifying the melted metal fed into the mold while forming a liquid well of the melted metal at the top of the mold,

drawing the solidified metal from the bottom of the mold,

continuously lowering a metal pipe having the same composition as the melted metal into the wall of melted metal while melting the lower end of the pipe therein to cool the surrounding melted metal in said well, and

insufflating an inert gas in said well through the metal pipe to further cool the surrounding melted metal in said well.

2. A continuous casting apparatus comprising a mold having an open upper end and an open lower end for receiving and solidifying melted metal therein,

a metal pipe of the same composition as the melted metal,

means for continuously feeding said pipe into said mold to cool the melted metal therein, and means for passing an inert gas through said pipe into the melted metal in said mold to further cool the melted metal therein.

3. A continuous casting apparatus as set forth in claim 2 wherein said first means feeds said pipe axially into said mold to cool the center of the melted metal. 

1. A process for the continuous casting of metal in a vertical mold comprising the steps of feeding melted metal into the top of the mold, at least partially solidifying the melted metal fed into the mold while forming a liquid well of the melted metal at the top of the mold, drawing the solidified metal from the bottom of the mold, continuously lowering a metal pipe having the same composition as the melted metal into the wall of melted metal while melting the lower end of the pipe therein to cool the surrounding melted metal in said well, and insufflating an inert gas in said well through the metal pipe to further cool the surrounding melted metal in said well.
 2. A continuous casting apparatus comprising a mold having an open upper end and an open lower end for receiving and solidifying melted metal therein, a metal pipe of the same composition as the melted metal, means for continuously feeding said pipe into said mold to cool the melted metal therein, and means for passing an inert gas through said pipe into the melted metal in said mold to further cool the melted metal therein.
 3. A continuous casting apparatus as set forth in claim 2 wherein said first means feeds said pipe axialLy into said mold to cool the center of the melted metal. 